The present invention relates principally to the electroless deposition of copper onto the suitably activated surfaces of a substrate, more particularly to the electroless deposition of copper onto the suitably activated through-hole surfaces of a printed circuit board substrate having through-holes, and still more particularly to process of this type where the through-holes have a high aspect ratio.
The electroless deposition of copper (here used to include copper alloys or intermetallics), i.e., the chemical plating of copper onto a catalytically activated substrate surface by chemical reduction without need for externally-applied electrical current, is a well-known process in extensive commercial use, particularly in the manufacture of printed circuit boards. Generally speaking, electroless copper depositing solutions (baths) are aqueous solutions containing a bath-soluble source of copper, a reducing agent for the copper ion, a complexing agent for the copper, and a source, if necessary, of acid or alkali to achieve the pH at which the bath is intended to operate. Typical baths are based upon formaldehyde (or a precursor thereof) as the reducing agent, but more recently baths have become available based upon hypophosphite reducing agents. See, e.g., U.S. Pat. Nos. 4,209,331 and 4,279,948 to Kukanskis, et al.
The substrate surfaces to be electrolessly copper plated are clean, catalytically active surfaces. Typically, the catalytic activation is by means of palladium/tin sols or solutions (see, e.g., U.S. Pat. Nos. 3,011,920 and 3,532,518) which often require an acceleration step to expose and/or activate the catalytic species. See, e.g., U.S. Pat. No. 4,608,275 to Kukanskis.
Generally speaking, the electroless copper plating process is carried out by immersion of the substrate into a tank containing the electroless bath. For the planar substrates typically employed in printed circuit manufacture, a number of such substrates generally are treated at one time, for example by racking them vertically in parallel in a suitable racking device which can then be serially immersed into tanks containing cleaning solutions, catalyst, accelerators, rinse tanks, and ultimately into a tank holding the plating bath. It is not uncommon to mechanically move the rack holding the substrates in a relatively slow back and forth motion perpendicular to the vertical disposition of the substrates in the bath as a means for improving plating uniformity over the substrate surface.
Among the most important uses of electroless copper plating is in the provision of the requisite conductive metal surface in the through-holes which are provided in double-sided and multilayer printed circuits and through which conductive interconnection is achieved between or among circuit patterns on the opposed substrate and/or innerlayer surfaces. The integrity of the conductive metal layer provided on these through-hole surfaces, in terms of its full coverage, adherence, and resistance to cracking or peeling under stress (particularly as will occur when components are later soldered to the printed circuit), is of decisive importance in achieving useful printed circuit boards. To this end, significant attention has been devoted to the preparation of the non-conductive through-hole surfaces for receipt of electroless metal, including steps for cleaning and desmearing the hole surfaces, altering their topography, conditioning them for adherent receipt of catalyst, and the like. See generally, Kukanskis, P., "Improved Smear Removal", Circuit Manufacturing, pp. 573-74 (March 1983); Kukanskis, P., "Improved Smear Removal Process For Multilayer Circuit Boards", IPC Technical Paper No. 435 (October 1982); U.S. Pat. Nos. 4,597,988 and 4,756,930 to Kukanskis, et al.; and Doubrava, J.J., "The `Black Hole` and Beyond: The Production Of Void-Free Plated Through Holes", PC Fab., August 1985. Work also has been devoted to particular formulations of electroless copper plating baths to provide deposits which are less resistant to stress and cracking. See U.S. Pat. Nos. 3,615,735; 4,228,213; and 4,695,505.
The integrity of the electrolessly deposited copper layer on through-hole surfaces is particularly important in so-called "additive" or "full build" printed circuit manufacturing processes in which electroless deposition provides the full extent of metallization in the through-holes (i.e., as opposed to processes in which only a thin electroless plate is provided which is then overplated with electrolytic copper).
For substrates having through-holes which have an aspect ratio ratio of substrate thickness to hole diameter) on the order, say, of 3:1, the conventionally-employed electroless plating techniques, e.g., involving slow mechanical back and forth movement of the racked substrates (in a direction normal to the vertical disposition of the substrates and, hence, parallel to the through-hole bore) have proven generally satisfactory for achieving good through-hole plating (providing, of course, that adequate hole preparation steps have been employed). Current design criteria for printed circuit boards, involving space-saving increased interconnect capability, have brought about multilayer boards having numerous innerlayer circuit patterns (thereby increasing the thickness of the board) while at the same time having numerous small diameter through-holes. Thus, aspect ratios ranging as high as 20:1 are encountered, and it has been found that this makes conventional electroless plating techniques increasingly less able to provide plated through-holes in which the electroless deposit possesses the necessary integrity. See generally, D'Ambrisi, J. J., et al., "The Chemistry Of Plating Small Diameter Holes--Part I", PC Fab, April 1989, and D'Ambrisi, J. J., et al., "The Chemistry Of Plating Small Diameter Holes--Part II," PC Fab, August 1989.
As is described hereinafter, our efforts initially directed to improved electroless plating of high aspect ratio through-holes have also led to significant and surprising findings applicable generally to electroless plating, including electroless plating of low aspect ratio through-holes heretofore considered satisfactorily plated using conventional techniques.